VFDs

A variable frequency drive (VFD) is an electronic device that controls the speed and torque of an alternating current (AC) motor by varying the frequency and voltage of the electrical supply. A typical VFD comprises a rectifier to convert AC input to direct current, a DC link, an inverter to synthesize AC output, and a controller that implements the selected drive algorithm. Modern drives usually employ a voltage-source inverter with insulated-gate bipolar transistors (IGBTs) and may include features such as active front ends, line reactors, and EMI filtration.

Operation and control: For most AC motors, speed is proportional to supply frequency, so VFDs adjust frequency to set speed while scaling voltage to maintain the appropriate volts-per-hertz ratio. This allows smooth acceleration and deceleration, reduced inrush, and torque control across a wide speed range. Control methods include open-loop voltage-frequency (V/Hz) control and closed-loop strategies such as vector control or direct torque control that use feedback from the motor (via encoders or resolvers) to improve low-speed torque and dynamic response. Some VFDs can operate in servo-like configurations for permanent magnet motors or provide regenerative braking to feed energy back to the supply.

Applications and benefits: VFDs are widely used in pumps, fans, conveyors, machine tools, and HVAC systems, particularly where load varies with process demand. Benefits include energy savings from speed matching, reduced mechanical stress through soft starting, improved process control, and extended equipment life.

Limitations and considerations: Not all applications benefit from a VFD, especially constant-speed duty or small motors where costs outweigh gains. Switching action introduces harmonics and EMI, potentially requiring filters or reactors. Proper sizing, cooling, and control integration are important, as is ensuring electrical safety and adherence to applicable standards and installation practices.